1. Field of the Invention
The present invention relates to an electric power steering control apparatus that generates assist torque for assisting steering torque of a driver by means of a motor.
2. Description of the Related Art
In general, an electric power steering control apparatus includes a steering torque detector for detecting steering torque generated by a driver, a phase compensator for improving the frequency characteristic of the steering torque, a torque controller for calculating an assist torque current for assistance of the steering torque based thereon, and a motor for generating an assist torque to assist the steering torque of the driver in accordance with the assist torque current.
In this case, the assist torque current calculated by the torque controller is a value substantially proportional to the steering torque from the steering torque detector of which the frequency characteristic is improved by the phase compensator. In addition, the greater a torque proportional gain set in the torque controller, the greater the assist torque current and the assist torque become, so the driver's steering torque is accordingly reduced.
If the torque proportional gain is set large, however, there occurs oscillation of a steering system, and vibration of a steering wheel becomes liable to occur, thus posing a problem of providing an unpleasant feeling to the driver.
Accordingly, in order to solve the above problem, a conventional electric power steering control apparatus includes a rotational speed sensor for detecting the rotational speed of a motor, a rotational speed high-pass filter (hereinafter abbreviated as a rotational speed “HPF”) for removing a speed component due to steering from the detected rotational speed of the motor, and a damping controller for calculating a damping current based on an output of the rotational speed HPF (see, for example, a first patent document: Japanese patent No. 3,712,876).
Here, by calculating the damping current based on the output of the rotational speed HPF, the vibration of the steering wheel is suppressed to reduce the steering torque without providing any unpleasant feeling to the driver.
In such a conventional electric power steering control apparatus, the damping current is calculated based on the output of the rotational speed HPF regardless of the steering state, and damping control for suppressing vibrations generated in the steering system is executed.
As a result, there has been a problem that in a state of holding the steering wheel (i.e., a state where the rotational speed of the motor is zero, and hereinafter being referred to as a “steering holding state”), or in a state of steering the steering wheel to a slight extent (i.e., a state where the rotational speed of the motor is low, and hereinafter being referred to as a “slight steering state”), the damping control becomes overcompensation, causing vibration of the steering wheel and providing an unpleasant feeling to the driver.